The present invention is an apparatus and method for displaying three-dimensional (3D) vectorcardiograms.
An electrocardiogram (ECG) is a measure of the electrical activity of the heart. It is typically measured by determining the voltage difference between a pair of electrodes placed on the body. However, because the body is a three-dimensional space, the actual ECG voltage is a three-dimensional vector in that space, and varies not only in magnitude, but also in direction. (Actually, the voltage is not even a simple vector, because it is not uniform throughout the body. Nevertheless, it is generally modeled as a vector.) Therefore, any single pair of leads measures only the voltage in the direction defined by the line between those two leads, and a set of three independent pairs of electrodes is required to measure the overall three-dimensional voltage vector. The three-dimensional ECG is usually referred to as a vectorcardiogram (VCG).
There exist a variety of schemes for displaying ECGs. See generally P. M. Rautaharju, "A Hundred Years of Progress in Electrocardiography 2: The Rise and Decline of Vectorcardiography," The Canadian Journal of Cardiology, 4(2):60 (1988). The simplest and most common is shown in FIG. 1. In this two-dimensional display the horizontal axis represents time and the vertical axis represents the ECG voltage. The ECG then describes a path determined by the variation of the voltage with time. Because this display contains only one voltage dimension, it shows only a one-dimensional projection of the actual three-dimensional ECG voltage.
An alternative display is the three-dimensional vectorcardiogram (VCG). The VCG is the path traced in (three-dimensional) space by the three-dimensional ECG voltage vector. A typical VCG is shown in FIG. 2, where the three dimensions are projected into a two-dimensional display plane. Although this display presents all three spatial voltages, it contains no explicit representation of time. Although the time history of the voltage can be observed dynamically as the spatial path is traced, the VCG, once displayed, shows only the relationships among the spatial voltages. It retains no information regarding the time history of those voltages.
Each of these two prior art display formats has its advantages. The typical ECG display (FIG. 1) explicitly shows time relationships, which allows characteristics of the heart beat cycle such as the P and T waves and QRS complex to be examined. Much of the expertise of medical professionals in examining ECGs derives from their familiarity with these particular figures and patterns in the ECG. Often, however, clinicians will use several ECGs, measured along different directions in the body, to recover some of the three-dimensional information lost in a single-voltage display. All of the information in these multiple ECGs is contained in the three-dimensional VCG display (FIG. 2), which additionally shows the spatial relationships among the various voltages. However, the lack in the VCG of an explicit representation of time makes it difficult for professionals using it to benefit from their experience in interpreting time-based ECGs.
What is needed is a display that relates the single-voltage ECG to the three-dimensional VCG to allow medical professionals to benefit from their prior ECG interpretation expertise. A prior art system to show the two displays side-by-side in a static format was described in W. R. Milnor, S. A. Talbot, & E. V. Newman, "A Study of the Relationship Between Unipolar Leads and Spatial Vectorcardiograms, Using the Panoramic Vectorcardiograph," Circulation, 7:545 (1953). This system allowed the display of what is basically a one-dimensional cross-section through a two-dimensional view of the three-dimensional VCG. Although the system therefore displays one relationship between the two displays, it is limited to display of that single relationship. Ideally, a display should be capable of dynamically showing the transition between the two display formats, so as to make their relationships apparent. Such a display would allow medical professionals to retain the benefit of their experience in interpreting ECGs without sacrificing the additional information available in the VCG. It would also provide an ideal tool for teaching and demonstrating the relationships among the spatial and temporal elements of the heart voltage. The present invention provides these benefits.